This invention relates to a communication system, a repeater terminal in a communication system and a communication method for repeating communication at the repeater terminal when packet data are transmitted from the transmitter terminal to a destination terminal.
Attention has been recently focused on the ad-hoc networks in which plural wireless terminals communicate each other without relying on particular infrastructure, e.g. base stations, wired networks. In ad-hoc networks, the networks are easily and flexibly formed by using direct communication between the terminals within a common communication coverage and a multi-hop communication through a terminal for repeating the communication between the terminals that cannot communicate each other directly.
For example, Japanese laid-open patent application Toku-kai No. 2003-209577 discloses an exemplary multi-hop communication technique for decreasing bit error rate and improving the communication efficiency by determining the size of transmitted packets depending on the characteristics of the communication line for transmission.
In general, it is required to implement a routing for determining the communication route by determining the terminal that the repeater terminal communicates next when a terminal (transmitter terminal) communicates with a remote destination terminal through another terminal. In an ad-hoc network using wireless, since the network information changes every second by movement of terminals, disconnection, etc., there are a lot of problems because of the complexity of the routing for an efficient communication between a transmitter terminal and a destination terminal.
In view of this situation, a technique for realizing a communication without routing in an ad-hoc network using wireless is proposed by sending ARQ (Automatic Repeat request) from the destination terminal to the transmitter terminal, and in response to the ARQ and simultaneously retransmitting from the transmitter terminal and the repeater terminals when an error occurred at the destination terminal.
For example, Japanese laid-open patent application Toku-hyo No. 2001-518725 discloses an exemplary technique relating to ARQ for combining the original signal that was retransmitted by ARQ with the obtained various kinds of information at the time when the signal was failed to receive.
In a circumstance requiring a multi-hop communication, a packet error occurs and the characteristics are significantly degraded if a normal multi-hop communication is implemented when the distance between the transmitter terminal and the destination terminal is large or when the power level drops due to a fading.
One of the techniques for improving the above mentioned degraded characteristics is known as an antenna diversity technique for transmitting packets using plural antennas. As one of the transmitter antenna diversity techniques, STBC (Space-Time Block Code) is known. This is a technique for obtaining a diversity gain on the receiver side by simultaneously sending the differently coded packets from the plural transmitter antennas individually. FIG. 7(A) shows an exemplary configuration of a STBC communication system using two transmitter antennas.
In FIG. 7 (A), the complex number signals (symbols) S0, S1, . . . are sent to a transmitter terminal 72 having STBC function from an information source 71. At the transmission terminal 72, a first STBC transmitting pattern (pattern 1: S0, pattern 2:−S1) and a second STBC transmitting pattern (pattern 1:−S1*, pattern 2: S0*) shown in FIG. 7(B) are generated from the consecutive two symbols S0, S1, and an antenna 731 transmits the pattern 1 signals S0, S1* and an antenna 732 transmits the pattern 2 signals S1, S0*. At a receiver terminal 74, a diversity gain which is equivalent to the Maximum Ratio Combination (MRC) can be obtained by performing a weighted combination as shown in FIG. 7 (A).
It is difficult to prepare plural antennas at one terminal to obtain a diversity gain in an ad-hoc network because miniaturization of terminals is required. A technique for obtaining a diversity gain is a STBC by utilizing terminals distributed in the surround area (distributed terminals) and simultaneously transmitting packets from the terminals. Each terminal is used as an antenna diversity branch. See, for example, Erina Kojima, Takeo Fujii, Yukihiro Kamiya, Yasuo Suzuki “Distributed ARQ using STBC for OFDM ad-hoc network”, Shingakugihou, June 2004, RCS2004-77, pp. 7-12. STBC has the advantage that the transmitter side needs no channel information and no phase sharing is required between the terminals when the distributed terminals transmit the signals. FIG. 8 shows a STBC model by plural terminals.
FIG. 9 shows a schematic view of signal transmission and reception by distributed terminal ARQ for an ad-hoc network using STBC, and FIG. 10 shows an exemplary signal transmission timing chart.
As a communication method, OFDM (Orthogonal Frequency Division Multiplexing) is used to reduce the effect of timing errors by GI (Guard Interval) considering the timing offset between terminals at the time of ARQ retransmission. FIG. 11 shows the operation of a transmission terminal S and a repeater terminal Rn (n=1, 2, 3, . . . ) at the time of packet transmission. When the transmission terminal S transmits packets to the destination terminal D (Step Sa1), the repeater terminal Rn that is in a waiting status (step Sb1) receives the packets (Step Sb3) and performs a provisional demodulation (Step Sb5).
When the demodulation was completed without errors (“Yes” at Step Sb7), the repeater terminal R waits for a packet retransmission (Step Sb9). When the demodulation has error (“No” at Step Sb7), the process returns to the reception waiting status at Step Sb1.
When the packet retransmission waiting time reaches a predetermined time (“No” at Step Sb11), the process ends. The transmitter terminal S determines whether it received ACK (Acknowledgement) from the receiver terminal D within a predetermined time (“No” at Step Sa3), the transmitter terminals S transmits a retransmission control signal (Step Sa7) when the number of packet transmission times is less than the predetermined number of retransmission times (“Yes” at Step Sa5).
The number of the transmitted retransmission control signals is set to identical to the number of packet retransmission times (r times). For example, if it is the second packet retransmission, two retransmission control signals are transmitted.
When it exceeds the predetermined retransmission times (“No” at Step Sa5), the packets are destructed and the process ends. The retransmission control signal includes the address of the transmitter terminal S, the address of the destination terminal D, the packet ID, the retransmission timing bit, the number of transmission times, maximum retransmission times, etc. Assuming that the retransmission control signal which is first received by the repeater terminal Rn is the N times transmission from the transmitter terminal S, when the present number of packet retransmission times is r (“Yes” at Step Sb13), the repeater terminal Rn checks the information included in the transmitted retransmission control signal, and updates the information for the number of transmission times that was saved in the repeater terminal Rn (Step Sb15). When the retransmission control signal includes the packet ID receives at Step Sb3 and maintained at the repeater terminal Rn, the repeater terminal Rn synchronizes the retransmission timing and transmits (r−N) times the control signal from the transmitter terminal S to the destination terminal D or another repeater terminal Rm (n≠m, m=1, 2, 3 . . . ) (Step Sb17).
Both the transmitter terminal S and the repeater terminal Rn which received the packets from the transmitter terminal S at Step Sb3 transmit the retransmitted packets to the destination terminal D or another repeater terminal Rm simultaneously (Steps Sa9, Sb19).
At this time, the transmitter terminal S and the repeater terminal R send two transmission patterns of STBC by selecting autonomously.
The repeater terminal Rm or the destination terminal D which received retransmitted packets from both the transmitter terminal S and the repeater terminal Rn using STBC separates the channel estimate values by the pattern of STBC, and performs a weighted combination and demodulation.
The transmitter terminal S returns to the operation at Step Sa3 and the repeater terminal Rn returns to the operation at Step Sb9.
FIG. 12 shows the operation of the destination terminal D and the repeater terminal Rm at the time of ACK transmission. The destination terminal D receives the packets (Step Sc1), and transmits a NACK to the transmitter terminal (Step Sc5) when the packets have an error (“Yes” at Step Sc3), the destination terminal D transmits a ACK to the transmitter terminal S (Step Sc7).
The repeating of the ACK and NACK is performed in the opposite way of the repeating of the control signal.
When the packets arrived at the destination terminal D by at the r th time retransmission the destination terminal D transmits (r+1) times ACK or NACK.
The repeater terminal Rm which received ACK or NACK at the M th time transmission (Step Sd1), the repeater terminal Rm repeats ACK or NACK to the transmitter terminal S by transmitting (r+1−M) times (Step Sd3).
Further objects and advantages of the invention will be apparent from the following description of the invention.